In recent years, solid state lighting devices, such as light emitting diodes (LEDs), have become widely used in a wide variety of industries and product lines. LED applications are now even becoming common as a replacement for traditional incandescent light bulbs.
However, LED light bulbs often create undesirable colors or color temperatures, as the light generated often appears as being “cool” or blue. Thus, attempts have been made to control the color of the light using phosphor in various ways. However, the use of phosphor to control color temperature of LEDs increases cost and reduces efficiency of solid state lighting. In addition, color temperature is not well controlled as operating conditions vary, such as when LED temperature and bias current change.
Other attempts have been made to control the color temperature by regulating current flow in a controlled manner in multi-string LED assemblies (e.g., having LED strings of different colors). For example, one method involves the use of a direct current-to-direct current (DC/DC) converter on the secondary side of a primary side controlled alternating current-to-direct current (AC/DC) flyback converter. However, such solutions are expensive due in part to the DC/DC converter and lack the flexibility to easily tune the circuit in production to account for lot-to-lot variations in LEDs and variations in other control circuit elements. In order to provide tuning, a micro-controller may be added. However, doing so even further increases costs.
Overall, the existing solutions are too expensive and too big for mass produced solid state light bulbs, especially for the retro-fit incandescent bulb replacement market.